Mined oil sand containing bitumen is generally slurried with a solvent such as water as part of an initial process for eventual removal of the bitumen from the oil sand. Oil sand is a type of bitumen deposit typically containing sand, water and very viscous oil (the bitumen). When the oil sand deposit is located relatively close below the ground surface, the oil sand is often extracted from the deposit by excavating down through the ground surface to where the oil sand deposit occurs and removing oil sand from the deposit with heavy machinery.
Clay lumps and rocks present in the oil sand ore body provide no value to an oil sands mining operator, however, the clay lumps and rocks must still be processed along with the oil sand lumps and, hence, are also introduced into the oil sand slurry. Unacceptably high bitumen losses result if oil sand slurry is screened prior to conditioning, as some of the undigested oil sand lumps will also be removed. Conditioning of oil sand slurry is conventionally achieved by hydrotransporting the oil sand slurry through a pipeline of a certain length, wherein lumps of oil sand are ablated or disintegrated, the released sand grains and separated bitumen flecks are dispersed in the water, the bitumen flecks coalesce and grow in size, and the bitumen flecks may contact air bubbles and coat them to become aerated bitumen.
However, during hydrotransport, not only do the oil sand lumps ablate, but the clay lumps also ablate, albeit at a slower rate, and the disturbed clay is the primary constituent of fluid fine tailings (FFT), which requires expensive post-treatment and re-handling. Further, it is also recognized that dissolving clay lumps are a significant constituent of the screened product (about ⅓ of the lumps are partly dissolved clays). Thus, excessive turbulence during the screening process can only have negative influence by creating more dissolved clays. Indeed, the downstream bitumen extraction process (e.g., gravity separation) is often impeded by ‘gel-like’ strata generated by the dissolved clays.
The rocks in the hydrotransport slurry also present negative value, since they damage piping and pump parts throughout the system, at a disproportionately high rate. Both the rock and clay lumps may increase bitumen losses in the coarse solids separation vessel, since they settle very quickly relative to smaller sand particles, and they are likely to drag down recoverable bitumen, and otherwise disturb the low-velocity zones required in any separation or desanding vessel that are required for high bitumen recovery rates.
Oil sand typically comprises about 85% solid material, and about 70% of the solids could be considered ‘coarse’. Of the coarse solids, 1-2% are above 50 mm in size, and an additional 2-5% are between 10 mm and 50 mm in size. Currently, the top size, set by screens or sizers, is set in order to avoid blockage of downstream equipment such as pumps. However, in existing systems, with front-end sizing, the oil sand lump size cannot be optimally reduced, resulting in either heavy loss of feed bitumen, or expensive reject re-processing systems.
It has recently been proposed to screen oil sand slurry after conditioning of the oil sand slurry, i.e., ablation of the oil sand lumps to a desired size, but prior to complete ablation of the clay lumps. However, screening a slurry comprising clay lumps and rocks poses certain challenges, as the clay lumps and rocks tend to impede the free drainage of the oil sand slurry.